First of all, you don’t have to call this method explicitly. It is called whenever you perform a subtraction operation. I.e., you can simply do:

let dt = dt - Duration::seconds(1);

And this should work.

If, however, you want to use the method explicitly, note that it doesn’t belong to the DateTime itself, but rather to the std::ops::Sub trait, and trait methods may be used only if the method is in scope. So, if you add